1. Field of the Invention
The present invention relates to a polycarbonate film and a production process thereof.
2. Description of the Related Arts
Polycarbonate film has been used in packaging, optical devices, display devices and other various applications in industry. Recently increasing attention has been paid to the film as a material for optoelectronic devices, and the practical application has been forwarded in retardation plates, polarization plates, plastic substrate, for example, in liquid crystal displays. Particularly, in the most remarkably progressing STN liquid crystal displays, the polycarbonate film is attracting attention as a phase-difference layer used in the space between the liquid crystal layer and the polarization layer for improving the image visibility, when it is seen from the front and oblique positions. This phase difference film plays a role of converting the elliptically polarized light passing through the liquid crystal layer into tile linearly polarized light, and the material is composed of a monoaxially oriented polycarbonate film mainly from bisphenol-A. The reasons why the film is used are that it has excellent properties which the phase-different plates are required for, for example, (1) high transparency, (2) optical anisotropy relatively easy to be controlled, (3) relatively high heat resistance, and (4) high uniformity.
But, the film of polycarbonate from bisphenol-A also has disadvantages. In general, this film is produced by casting a bisphenol-A polycarbonate solution in methylene chloride in order to attain the high level of uniformity, but the film for phase different plates or liquid crystal base plates is thick and requires to be cast from dope of high polymer concentration. But, the solubility of the polymer is about 20% by weight, not always adequately high. Moreover, the polymer cannot be stable in the dope of high concentration, and frequently causes cloudiness or gelation due to crystallization with the passage of time, especially the crystallization occurs during the film-making process in many cases.
Further, the film used in these optoelectronic devices is required as an electronic part not to give adverse effect on the surrounding parts, for example, not to develop chloride ions by decomposition caused by exposure to the atmospheres of high temperature and high humidity for a long period of time. Customarily, the cast polycarbonate film from the solution in methylene chloride is used for such an optical purpose, as mentioned above, but it is difficult to say that this polycarbonate film satisfactorily meets these requirements.
In the film for phase different plate and liquid crystal base plate, a high degree of surface flatness and controlled orientation are required, but it is not easy to give the products of high quality, satisfying these requirements from the above-mentioned solution in methylene chloride.
In the production of such film, the casting process where a film-forming solution is dried is employed instead of the extrusion process in order to maintain the uniformity. When the solution (dope)is formed, halogenated hydrocarbon such as methylene chloride has been mainly employed in high frequency. But, these solvents include halogen and liberate hydrogen halide, typically hydrogen chloride, by reaction with moisture in air, while the solvent component is evaporated and vaporized off, and the hydrogen halide often causes corrosion of the base plate and environmental problems, for example, deterioration of working environments.
Further, trace amounts of halogen elements such as chlorine remains in the film or base sheet which is made by casting the dope in a halogenated solvent. The adverse effects of remaining halogen on ITO (indium oxide) constituting the transparent electrodes and the components constituting TFT (thin film transistor) are concerned. The anxiety becomes more serious, particularly when the film is formed by casting a polycarbonate solution in a halogen-containing solvent boiling at a high temperature.
Moreover, the solvent such as methylene chloride or tetrachloroethane has high affinity for polycarbonate, and shows a tendency of increasing viscosity of the solution. Thus, the troubles in handling are caused because the solution viscosity becomes extremely high, even when the solutions of the same concentration are prepared. Conversely, in order to keep the viscosities equal, it becomes disadvantageously difficult to dissolve polycarbonate of high polymerization degree into a solution of a favorable concentration for film making.
When the resin solutions of a polymer are prepared from different solvents, the viscosities are governed by the interaction between the polymer and the solvents. In general, the solvent giving the solution of a higher viscosity has higher affinity to the resin than the solvent giving a low viscosity and the resin distributes in a linearly extended form in which the molecules tend to cause entanglement, while in the low-affinity solvent, the molecules shrink in a round form to lower their entanglement. Usually, polycarbonate solution in methylene chloride has a high viscosity and the film cast from the solution tends to be orientated, resulting in a defectively large retardation.
In the meantime, it is also possible to prepare a dope by dissolving polycarbonate in tetrahydrofuran, a halogen-free solvent, and a process can be imaged for casting the dope on a suitable support and giving thin film by evaporating the solvent to dryness. But, it cannot be said that tetrahydrofuran is a suitable solvent for polycarbonate. For example, the polycarbonate solution in tetrahydrofuran changes into gel with the passage of time, when the concentration exceeds 10% by weight, although the tendency depends upon the molecular weight of the resin. In other words, at first the increase of apparent solution viscosity is noticed, then occurrence of turbidness and gel formation with complete turbidness are observed, and finally the fluidity is lost. Thus, practical application of the dope has met serious problems. Further, the film of a large thickness cast from the solution in tetrahydrofuran frequently develops cloudiness on the way of drying.
As above-mentioned, the solution in tetrahydrofuran has a problem on its stability, while methylene chloride which has been mainly used as a solvent for polycarbonate has environmental problems. In other words, methylene chloride is possibly carcinogenic and may be regulated in the near future. Moreover, methylene chloride boils at a low temperature (boiling point: 41.degree. C.) and easily vaporizes off from the film-making system, thus doubling the environmental pollution.
Therefore, the use of dioxane has been proposed as a substitute for tetrahydrofuran in Japan Patent Specification Laid-open No. Tokkai-Hei 2-227456. Dioxane is, however, a cyclic ether bearing ether linkages in 1-, and 4-positions and their polarities are canceled by each other and it can be presumed that there is a limit in the ability to dissolve a polar polymer such as polycarbonate. Actually, polycarbonate is not dissolved in dioxane in the concentration higher than 20% by weight in many cases.